Field of the Invention
The present invention relates to methods and systems for improving surgical procedures for improving incomplete full or partial thickness corneal incisions.
Discussion of Related Art
Presently, there are a number of surgical methods for correcting maladies of the eye that involve forming an incision in the cornea of the eye. For example, it is known to surgically correct astigmatism by forming limbal relaxing incisions (LRIs) in the eye, wherein such LRIs are generally paired arcuate incisions/cuts formed in the cornea of the eye. In the past, such incisions were formed manually with a fixed or variable depth blade.
Recently, the practice of making the incisions manually with the above mentioned fixed or variable depth blade is starting to be supplanted by incisions made with a femtosecond laser (Maxine Lipner, EyeWorld, “What's Ahead, Femtosecond technology changing the cataract landscape”, 2011-3-24 8:45:27). Such a laser makes incisions by focusing ultrashort laser pulses to a very fine focus, causing a plasma mediated photodisruption of the tissue at the point of focus. An incision is generated by placing a contiguous series of such pulses in a pattern that results in the formation of the desired incision. To make a corneal incision, the point of focus of a femtosecond laser is scanned across a planar or curved surface within the volume of the target tissue to form the incision. The beam intensity at the focus is chosen to substantially exceed the laser induced optical breakdown threshold of the tissue. As each pulse is delivered, a plasma-mediated photo-disruption occurs, vaporizing a miniscule volume of tissue at or near the point of focus. A cavitation bubble subsequently forms near the point of focus which helps cleave the damaged region to form the incision. Using a scanning laser guidance system, laser pulses are placed contiguously in three dimensions across the desired planar or curved surfaces to form the overall incision. The combined effect of the pattern of pulses is to cleave the tissue at the targeted plane. Arbitrarily complex incisions patterns can be generated with such lasers. The femtosecond lasers are believed to make incisions of a more accurate and consistent depth and of a curvature that more accurately matches the desired arcuate form of the incision.
There can be circumstances where the above mentioned femtosecond laser generates a low numerical aperture (NA) (or slow F-Theta lens) laser beam and is paired with a liquid patient interface. A comparison between a high numerical aperture laser beam 100 that passes through a liquid patient interface 102 and a low numerical aperture laser beam 104 that pass through a liquid patient interface 102 is shown in FIGS. 1A and 1B. As shown in FIG. 1A, a high numerical aperture laser beam 100 passes through a liquid patient interface 102, resulting in a focused high numerical aperture laser beam 106. The focused high numerical aperture laser beam 106 is directed into a portion 108 of the anterior corneal surface of the cornea of the eye and the beam 106 reaches the rear portion 110 of the portion 108.
As shown in FIG. 1B, a low numerical aperture laser beam 104 passes through a liquid patient interface 102, resulting in a focused low numerical aperture laser beam 112. The focused high numerical aperture laser beam 112 is directed into a portion of the anterior corneal surface 108 of the cornea of the eye and the beam 112, the corneal entry incision leaves an unintended residual thin but uncut layer 114 at the anterior corneal surface, such as at the Bowman's membrane of the cornea which has the stiffest collagen fibers and at the posterior corneal surface, such as Descemet's membrane which is relatively softer. The formation of the unintended uncut layer 114 is due to the fact that there is a difference at the interface between the optical breakdown thresholds of the epithelium layer of the cornea and the Bowman's membrane of the cornea at the anterior corneal surface of the cornea of the eye. Similarly, an uncut layer at the posterior corneal surface is formed due to the optical breakdown thresholds of the endothelium layer of the cornea and the Descemet's membrane of the cornea at the posterior corneal surface of the cornea of the eye. An example of such an unintended uncut layer is shown in FIGS. 2A-B and 3A-B. Note that the unintended uncut layer can be generated in a variety of incisions. For example, uncut layers 116, 118 of FIGS. 2A-B and 3A-B can be generated in a so called Full Thickness corneal Incision (FTI), which is an intended incision from posterior to anterior surface of the cornea as would be the case for Clear Corneal Incisions (CCIs), paracentesis incisions or Penetrating Keratoplasty (PKP) or other through surface modalities. As another example, the uncut layer can be generated in a so called Partial Thickness Incision (PTI), which intentionally starts within the stroma and progresses through the anterior surface of the eye as would be the case for Limbal Relaxing Incision (LRI) and Astigmatic Keratotomy (AK) or other partial thickness modalities. In either example, the presence of the uncut layer results in an incomplete FTI or PTI incision being formed. Note that in either the FTI or PTI incision, the thickness of the residual uncut layer can vary from approximately 10 μm to approximately 30 μm, as a function of the numerical aperture and the output energy of the laser beam.
One shortcoming of the incomplete full or partial thickness corneal incisions of FIGS. 2A-B and 3 A-B is that it can be relatively difficult to locate and open the wound due to the strength of the residual thin layers 116, 118 left uncut. For human eyes, the residual uncut layers encompass the Bowman's membrane, for partial thickness corneal incisions, and, Bowman's and Descemet's membranes, for full thickness corneal incision. Bowman's and Descemet's membranes are the regions of the eye structure with the stiffest collagen fibers.